1. Field of the Invention
The invention relates to a method for connecting a flexible flat cable using an ultrasonic welding machine and to a horn of the ultrasonic welding machine.
2. Description of the Related Art
A flexible flat cable has a conductive element, such as a copper foil with a thickness of about 35 xcexcm. The foil is covered by an insulation coating layer made, e.g. of a PET (polyethylene terephthalate). The flexible flat cable can be connected to a member, such as a busbar, by ultrasonic welding. More particularly, the conductive element is exposed by stripping the insulation coating layer over a specified range. The flexible flat cable and the member to be connected then are placed between an anvil and a horn of an ultrasonic welding machine so that the exposed surface of the conductive element contacts the upper surface of the member to be connected. An ultrasonic vibration is given to the horn while the flexible flat cable and the member to be connected are pressurized, thereby bringing metal atoms of the conductive element and the member to be connected into contact and connecting the two by an inter-atomic attraction.
The base material of the conductive element has a fairy low strength, and the strength of the base material tends to decrease due to damage caused by welding with the ultrasonic welding machine. Thus, it is difficult to ensure sufficient strength for a connecting portion of the conductive element of the flexible flat cable and the busbar or the like. There has been a problem that the conductive element of the flexible flat cable will break at the connecting portion, for example, due to a tensile force on the flexible flat cable.
Japanese Unexamined Patent Publication No. 2000-294332 discloses a process of connecting flexible wires to form conductive patterns while being held between a pair of insulation coating layers. One side of each insulation coating layer is removed to expose one surface of each wire. The stripped portions then are disposed between an anvil and a horn of an ultrasonic welding machine with the exposed surfaces of the wires put together and held in contact. An ultrasonic vibration is transmitted from the horn to conductive elements via the insulation films while the stripped portions are pressurized. Thus, ultrasonic welding is applied to the conductive element of one flexible wire and that of the other flexible wire to connect the two flexible wires while the outer surfaces of the conductive elements are covered by the insulation coating layers.
The above-described process connects the conductive portions while the outer surfaces of the connecting portion are covered by the insulating coating layers. The breaking strength of the connecting portion against an external load logically should be improved by the reinforcing action of the insulation coating. However, the ultrasonic welding is applied by transmitting the ultrasonic vibration to the horn while the horn is pressed in contact with the insulation coating that covers the outer surfaces of the conductive elements. Vibration energy applied during ultrasonic welding is absorbed by the insulation coating. As a result, proper ultra sonic welding of the conductive elements is difficult, and the conductive elements ultra sonically welded by this process have a low break strength.
FIGS. 11 and 12 show an ultrasonic welding machine 30 with a horn that has truncated pyramidal protrusions 32 that are intended to improve the breaking strength of ultra sonically welded parts. The protrusions 32 are formed at a specified interval on the bottom surface of the horn 31. The ultrasonic welding machine 30 also has an anvil 33. A flexible flat cable 1 and a member to be welded, such as a busbar 2, are introduced between the horn 31 and the anvil 33 of the ultrasonic welding machine 30. The protrusions 32 of the horn 31 bite into an insulation coating layer 5 that covers the outer surface of the flexible flat cable while the insulation coating layer 5 is molten by vibration energy transmitted to the horn 31. A conductive element 4 of the flexible flat cable is welded ultrasonically to the busbar 2 or the like with the leading end surfaces of the protrusions 32 pressed in contact with the conductive element 4 of the flexible flat cable 1.
Coating material that is molten during ultrasonic welding may clog the spacings between the protrusions 32, and may cause a connection failure. Thus, leading end surfaces of the protrusions 32 cannot be pressed into contact with the conductive element 4 when the coating material clogs the spaces between the protrusions 32. Thus, the vibration energy transmitted to the conductive element 4 decreases and proper ultrasonic welding cannot be performed.
In view of the above, it is an object of the invention to enable a flexible flat cable to be connected easily and properly to a member such as a busbar.
The invention relates to a flexible flat cable connecting method for welding a flexible flat cable. The flexible flat cable has a conductive element made of, e.g. a copper foil that is covered by an insulation coating layer. The connecting method employs an ultrasonic welding machine to connect the flexible flat cable with a member to be connected. The method comprises stripping the insulation coating layer from a connecting surface of the flexible flat cable to expose the conductive element. The method then includes introducing the flexible flat cable and the member to be connected between a horn and an anvil of the ultrasonic welding machine so that the connecting surface of the flexible flat cable is held in contact with the member to be connected. Elongated projections are provided on a press-contact surface of the horn. The projections have a tapered cross section and preferably are pressed in contact with the insulation coating layer of the flexible flat cable. The method then comprises transmitting an ultrasonic vibration to the horn. Thus, the elongated projections bite into the insulation coating layer to ultrasonically weld the conductive element to the member to be connected.
Accordingly, a connection strength between a flexible flat cable and a member to be connected such as a busbar is improved since a vibration energy can be transmitted efficiently to the conductive element while the leading ends of the elongated projections are pressed in contact with the conductive element. Moreover, the overall strength of the connection can be improved by the remaining insulation coating in the connection area.
The elongated projections on the press-contact surface of the horn preferably extend substantially in the longitudinal direction of the flexible flat cable.
Ultrasonically welding is repeated by causing the elongated projections of the horn to bite in the insulation coating layer of successive flexible flat cables. Coating material molten by the vibration energy during the first ultrasonic welding does not adhere to the press-contact surface of the horn and hence does not clog spaces between the elongated projections. Thus, vibration energy is transmitted efficiently to the conductive element during the next of the ultrasonic welding because the leading ends of the elongated projections are pressed in contact with the conductive element. Therefore, the conductive element can be welded ultrasonically to the member to be connected, while the outer surface of the conductive element is covered by the insulation coating layer and the conductive element and the member to be connected can be connected securely.
An exposing step of the method may further comprise partly stripping the insulation coating layer at a side opposite from the connecting surface of the flexible flat cable to expose the conductive element. With this method, the operation of ultrasonically welding the conductive element to the member to be connected can be repeated at the non-stripped portion of the insulation coating layer at the outer side of the flexible flat cable by causing the elongated projections of the horn to bite in the insulation coating layer of the flexible flat cable. A vibration energy then can be transmitted efficiently to the conductive element while the leading ends of the elongated projections are pressed in contact with the conductive element. However, the coating material molten by the vibration energy at the time of ultrasonic welding will not adhere to the press-contact surface of the horn to cause clogging between the elongated projections. Therefore, proper ultrasonic welding can be achieved while the outer surface of the conductive element is covered by the insulation coating layer and the conductive element and the member to be connected can be connected securely. Further, the conductive element can be connected
Securely with the member to be connected by performing ultrasonic welding at the stripped portion of the insulation coating layer at the outer side of the flexible flat cable with the press-contact surface of the horn directly pressed in contact with the conductive element.
Slits preferably are formed in the elongated projections on the press-contact surface of the horn to make the elongated projections discontinuous along the longitudinal direction of the flexible flat cable.
The length of the stripped insulation coating layer at the side of the connecting surface preferably is slightly larger than an entire length of the horn.
The invention also relates to a horn of an ultrasonic welding machine for ultrasonically welding a flexible flat cable. The horn has a press-contact surface to be pressed into contact with the flexible flat cable. The press-contact surface has plurality of elongated projections having a tapered cross section. The elongated projections preferable are arranged to extend substantially in the longitudinal direction of the flexible flat cable.
With this construction, the ultrasonic welding can be repeated without having the molten coating material adhere to the press-contact surface of the horn in a manner that would clog spaces between the elongated projections. Therefore, the conductive element can be welded properly to the member to be connected while the outer surface of the conductive element is covered by the insulation coating layer.
Slits preferably are formed in the elongated projections on the press-contact surface of the horn to make the elongated projections discontinuous preferably substantially along the longitudinal direction of the flexible flat cable.
With this construction, recesses corresponding to the elongated projections are formed in a connecting portion of the conductive element of the flexible flat cable and the member to be connected, and discontinuous portions that make the recesses discontinuous are formed to correspond to the slits. Thus, a breakage created by a tensile load on the flexible flat cable is prevented from progressing along the recesses, and the conductive element of the flexible flat cable and the member to be connected can be held connected.
The discontinuous elongated projections preferably have an extension or length between about 0.3 mm and about 1 mm, more preferably between about 0.4 mm and 0.7 mm.
The press-contact surface of the horn may have an elongated projection area and a protrusion area. The elongate projection area has a plurality of elongated projections to be held substantially in contact with the non-stripped portion of the insulation coating layer. The protrusion area has a number of protrusion of tapered cross section to be held substantially in contact with a stripped-portion of the insulation coating layer.
With this construction, the stripped portion and the non-stripped portion of the insulation coating layer are provided on the outer side of the flexible flat cable. The ultrasonic welding of the conductive element to the member to be connected may have to be repeated. Thus, the elongated projections bite in the non-stripped portion and transmit vibration energy efficiently to the conductive element while the leading ends of the elongated projections press in contact with the conductive element. The coating material molten by the vibration energy will not adhere to the press-contact surface of the horn and will not clog spaces between the elongated projections. Further, the conductive element can be connected securely with the member to be connected by ultrasonic welding the stripped portion of the insulation coating layer with the press-contact surface of the horn directly pressed in contact with the conductive element.
A projecting distance of the protrusions preferably is longer than a projecting distance of the elongated projections by a distance corresponding to about the thickness of the insulation coating at the non-stripped portion.
The elongated projections preferably have a substantially acute-angled isosceles trapezoidal cross section.
These and other objects, features and advantages of the present invention will become more apparent upon reading of the following detailed description of preferred embodiments and accompanying drawings. It should be understood that even though embodiments are separately described, single features thereof may be combined to additional embodiments.